This book constitutes the refereed proceedings of the Third International Conference on Persuasive Technology, PERSUASIVE 2008, held in Oulu, Finland, in June 2008. The 17 revised full papers and 12 revised short papers presented together with 3 keynote papers were carefully reviewed and selected from 63 submissions. The papers are organized in topical sections on social network systems, knowledge management, applications, conceptual frameworks, perspectives on persuasive technology, peer-to-peer and social networks, self-persuasion and timing, well-being applications, and theoretical considerations.